Gas, Existence Of
Before the seventeenth century, people thought that air was the only gas that existed. In fact, air was called an "element," along with water, earth, and fire. First proposed by Greek philosophers in ancient times, this concept was accepted for nearly 2000 years. The discovery and study of gases during the seventeenth and eighteenth centuries mark the beginning of chemistry as we know it today.
The first scientist to distinguish specific gases from ordinary air was Johannes Baptista van Helmont, who coined the term gas around 1620. Helmont applied scientific techniques to study "gas sylvestre," which is now known to be carbon dioxide. Although he also described several other gases, Helmont lacked the experimental equipment needed to collect gases in containers and study their qualities. In the 1650s, Robert Boyle became the first scientist to collect a gas ( hydrogen), using an inverted, submerged flask. Stephen Hales later improved upon Boyle's method and developed other devices for collecting and measuring gases.
In the 1750s, Joseph Black proved experimentally that carbon dioxide, which he called "fixed air," was distinct from ordinary air. Black realized that gases must be weighed and accounted for in chemical reactions. His research opened the door to the discovery and analysis of many new gases during the late 1700s, including hydrogen, oxygen, and nitrogen. Joseph Priestley advanced the study of gases by developing techniques for collecting gases over mercury instead of acids or water. Using this new method, Priestley isolated such gases as ammonia, sulfur dioxide, and carbon monoxide.
The true nature of gases, however, remained obscure, as many scientists persisted in calling gases "airs" because they were thought to be air contaminated with other substances. Finally Antoine-Laurent Lavoisier, building upon Black's analytical work and the ideas of French economist and philosopher Anne-Robert-Jacques Turgot (1727-1781) on the expandability of air, proposed that gases are actually a third state of matter. Unlike solids and liquids, gases expand to fill the size and shape of whatever contains them. Lavoisier also clarified the basic process of combustion, which consumes oxygen, and began to assign gases their modern names.
During this period of discovery, scientists also began to understand and explain the behavior of gases. As early as 1662, Boyle had developed an explanation, Boyle's law, for the relationship between the volume of a gas and its pressure, which are inversely proportional. Much later, in 1802, Joseph Gay-Lussac explained the relationship between gas volume and temperature, which became known as Gay-Lussac's law. He also found that gases combine with each other in simple volumetric proportions, a phenomenon that Amedeo Avogadro explained when he hypothesized that equal volumes of gases contain equal numbers of molecules (as long as the gases are at the same temperature and pressure). During the 1850s, scientists applied these laws to determine the molecular formulas and atomic weights of gases and to develop a kinetic theory of gases that explains their behavior in fundamental physical terms.
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